Method of making inlaid circuits



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Patented Oct. 19, 1954 UNI TED `S TA'eE'ES v v 2,692,190 vMILZEH'OD .0F MAKING CIRCUIT-S y Nathan ritikin, chicago, ,111. itpplicat-ion August-17, 1953, SerialNo5374i603 .13 Claims. (Cl. 41-1-42) Thisfinvention relates to .a method `of :making inlaid circuits, :and it is fanlobject of the invention to :provide animproved method of Vthat character.

it is common .practice .in 'the manufacture of large `cgiuantites vofrduplic'ate circuits .to employsocalled printed fcircuits. AIt is `desirable in vinost cases, zthat :such .printed circuits :have a flush surface, lthat is, :that lthe electrical tconductors fbe embedded fin lthe `insulating ibase ,material Aso that .the exposed :surfaces of 'the conductors are flush .with that :of the base. when :movable electrical contact is to abe made with portions'of .-thercircuit. Most'commonly the electrical conductors are first :laid `,on a tempo- .tary base yby lprinting, 'by .an `etched foil process, or 'by an electrolytic process. Subsequently, the material which is 44,to constitute the permanent base is pressed against the :exposed lsurfaces of ethe conductors and against :the exposed surface o'ffthe temporary base. Following this theprinted fcircuit, that is, kthe permanent 4base rvand the -embedded conductors, `'is stripped .from the `:tempo- Arary base. VIneorder to .permit this last-step .it -is necessary, vwhen electrolytically depositing the .circuit on .a temporary base, totreat the-surface of fthe cathode, which `serves as the temporary fbase, 4by anyone of Yseveral -known methods, `to prevent too :rm a `bond between v.the temporary base fand the deposited metal. `For example, an oxide may `rbe fpermittedorencouraged toiorm on -the surface `of the cathode or .a very .thin .ilm .of -grease vmay-be applied. In either zcase, .the .interivening Vforeign material permits the deposited smetal'tofbe stripped .fromthe .temporaryibase very readily, fand-atthe Sametime is suiliciently thin `thatthe applied voltage-may act acrossthe bar- -rier fiilm to cause `rreasonably uniform deposition 'ofmetal.

With the'methods presently being-employed Ain making inlaid circuits, considerable difficulty `is -experienced :in obtaining -a truly `iush surface. -One tfault 'which commonly `occurs is thatthe xmaterial which constitutes the permanent base -for itheicircuit flows partiallyover .the surfaceofthe .circuit .which adheres only flightly 4to 4,the cathode aand which -is :intended ato tbe Aexposed in the L comipleted'product. This common'fault results from the fact fthat :the material lconstituting the acir- =c.uit fis fnot :bonded to the iteinporaryfbase with suicient adhesion or cohesionto 4preventthema-- iterial cconstituting the :permanent base from seeping :between :the circuit :materialland ithe temporary base. YThis is particularly Itrue where 'the material Vconstituting :the permanent Arbase ris vfaithermosetting plasticwhichiisicuredrunder sub- This is particularly truey fr ortica stantial :pressure as well as a high temperature. Whenthis fault occurs, the -inlaid'circuit becomes areject. The particular portion of the `circuit improperly -covered by the base ymaterial may, vfor example, be `intended to `be exposed in order that electrical contact vcan be made therewith. Also, the portion 'of the printed circuit which is lifted on the face of the temporary base by the lpressures of the base `material ymay be .distorted to the extent that -a high resistance area-or even an ,open circuit fis produced.

According to the present invention the Vprinted circuit is bound `to the temporary base so rmly .that ffor all ,practical ,purposes it cannot be removed. More specifically, ,the material of the printed circuit becomes substantiallyonepiece of metal with the temporary base vwith the result .that no method-employed for-applying the perassured.

It is another object of the invention to provide an improved method .of ,producing inlaid circuits in which .the printed-.circuit is bound to the temporary base so firmly as .to `eliminate .the seepage of the material of .the .permanent `base between the printed ,circuit andthe temporary base.

`It isanotherobject of .the invention to provide yanimproved method -of Vproducing inlaid circuits vwhich vhas the Aadvantages specied above while bei-ng .accurate, reliable, V.and economical to yem- This invention, together with `further objects and advantagesthereof, will best be understood by'reference to thefollowingdescription taken in .connection .with the accompanying drawing and its scope will be pointed `out .in the appended fclaims.

v In the drawingin Whichlikeparts are designated by .like reference numerals,

Fig. 1 is a cross-sectional'view of a temporary .base employed .in one embodiment of the/invention;

Figs. 2, A3, -4,'and A5 arefviews similar 'to Fig. -zl

-but illustrating succeedingfsteps.l in the formation of aprinted circuit onthe temporary base in accordance .with one embodiment of .the invention;

Fig. 6 is a similar View showing a permanent lbaseadded to the assembly of Fig.5;

jFig. Jl' is la `viewsimilar to Fig. 6 but showing the temporary baseremoved; and

Fig. 8 is a plan view of a printed circuit which may be produced in accordance with the present invention.

The embodiment of the invention illustrated in the drawings discloses the invention as applied to a resistance element which may be employed in the construction of a potentiometer. It is to be noted that the term inlaid circuit, as employed herein, is to be interpreted in its broadest sense, whereby it includes not only a maze of conducting strips which constitute the conventional idea of a printed circuit, but also such simple arrangements as the resistance element disclosed in Fig. 8 and even simpler arrangements. More specilically, the term is intended to include any oonducting element embedded in an insulating material for the purpose of carrying electricity.

In the embodiment of the invention illustrated in the drawings a temporary base I I is employed which preferably constitutes a thin copper sheet, namely, one on the order of five mils thick. A sheet of this thickness permits convenient handling of the assembly during process. However, a heavier sheet may, if desired, be employed, and a very thin lm or foil may be employed, but in the latter case the foil is preferably backed up by a relatively stii base to which the film adheres lightly. Y

In accordance with the preferred embodiment of the invention, the copper sheet I I is to be used as a cathode upon which the conducting material of the ultimate printed circuit is electrolytically deposited. Since the conducting material is to be deposited in a desired configuration, it is necessary that some form of insulating resist be applied to one surface of the copper sheet. In accordance with the preferred embodiment of the invention, this is accomplished photographically.

In Fig. 2 a photosensitive coating I2 is shown applied to one surface of the copper sheet II. This photosensitive coating may, for example, be a solution of shellac sensitized by a bichromate solution. Since a photosensitive coating of this character is commonly employed in the art, it is not described in detail herein.

The photosensitive coating is exposed to activating rays, such as light, in a pattern which is the negative of the desired ultimate configuration of the conducting material of the printed circuit. After exposure the photosensitive coating is developed and fixed in the manner normally employed in treating this common form of photosensitive coating. As a result of this process the nonactivated portions of the photographic emulsion are washed away, leaving a residue coating I2 in a pattern which is the negative of the desired ultimate configuration of the conducting material of the printed circuit (see Fig. 3). Preferably, this residue coating I2 is subjected to a temperature on the order of 250 F. for fifteen minutes to drive out moisture and to set the coating. This makes the coating more durable and thus better able to stand up during subsequent operations.

In accordance with this invention, conducting material for the printed circuit is to be bonded to the base sheet II with the tightest possible, direct metal-to-metal bond. Accordingly, at least the remaining exposed portion of the lower surface of the sheet II, in Fig. 3, is made chemically clean. This may be accomplished by dipping the assembly of Fig. 3 in a 10% solution of sulphuric acid. It should be noted that this bath will not destroy the residue coating I2'. Y

According to the preferred embodiment of the invention, the entire assembly of Fig. 3 is then immersed in an electrolytic bath containing a silver salt, and silver is electrolytically deposited on the exposed portions of the lower surface of the copper sheet II. During this process the reverse or upper side of the copper sheet may be protected by an over-all coating of any suitable nonconducting material in order to prevent deposition of silver on the reverse side.

The silver so deposited on the exposed portions of the lower surface of the copper sheet then constitutes the conducting portions I3 of the ultimate printed circuit and is in the desired configuration (see Fig. 4).

Since copper is electropositive with respect to silver in the electromotive series, silver may be chemically deposited on the copper prior to the application of voltage necessary for electrolytic deposition. To avoid this chemical deposition, which tends to be coarse and structurally weak, a thin lm of silver, on the order of .l mil thick, may be electrolytically deposited from a strike solution before employing the conventional electrolytic bath. Such a strike solution is very low in silver as compared to the conventional electrolytic bath and, accordingly, substantially no silver is chemically deposited on the copper from this solution. Since the use of a strike solution is common in electrolytic deposition of metals, this step is not described in detail herein.

The residue coating I2 may then be removed by treating with a suitable organic solvent such as alcohol or an alkaline solution of lye, leaving only the deposited metal I3 on the sheet II as in Fig. 5. These agents have no detrimental effect upon either the copper sheet II or the electrolytically deposited silver I3.

Next asuitable insulating material I4 which is to form the permanent base of the inlaid printed circuit is applied against the deposited silver and against the exposed portions of the lower surface of the copper sheet II, as in Fig. 6. This material should be in plastic form in order that it may be made to come into intimate contact with all exposed surfaces. Preferably this material is a plastic such as Teflon or polystyrene, but may be any thermoplastic or thermosetting plastic or any hardenable insulating material suitable for the purpose. During this step the copper sheet II is permitted to lie against a flat rigid base. Where thermosetting plastics are employed as the permanent base I4 they may at this point be thermally set by application of elevated temperatures suitable for the particular thermosetting plastic employed. Preferably a substantial pressure is also applied in order to insure firm and uniform contact between the plastic and the exposed surfaces of the copper and the silver. The application of pressure is desirable for this reason regardless of the material used for the permanent base.

After the base I4 has been thermally set or otherwise brought to its final form, the copper sheet II is etched away to expose the surfaces of the silver I3 and the base I4 which were previously in contact with the copper sheet, as seen in Fig. 7. In the specic embodiment of the invention described above, namely, where a copper sheet II is employed along with a silver deposition I3 and any form of thermosetting plastic for the base I4, the copper sheet may be removed by a 40 Baume, water solution of ferric chloride.. This agent readily dissolves the mosaico 5 copper sheet and has lsubstantially no effect upon the Silver l' the DeImahElI-t baise.

'The removal of the base sheet l I from the inlaid printed circuit by a solvent for that sheet which is substantially inert toward the materials of the inlaid circuit, is practicable regardless 'of the nature of the bond between the base sheet and any of the materials of the inlaid circuit. Accordingly, the nrmest'possible metal-tometal bond is permitted between the conducting portions of the inlaid circuit and the base sheet. More specifically, where the conducting materials of the inlaid circuit are electrolytically doposited on Ythe base sheet, the surface of the base sheet may be, and preferably is, chemically lclean at the time of the electrolytic deposition, vwhereby the conducting lportion of the inlaid circuit vand the base sheet become substantially a `single piece of metal. f

Where this process is employed the insulating material which is to form the permanent base of the inlaid circuit may be applied to the rexposed surfaces of the Ametal sheet and of the electrolytically deposited metal under the severest conditions of pressure land temperature without fear that the insulating material may seep between the base sheet and the conducting material which has been electrolytically deposited thereon. Also, there is no possibility that any portion ofthe conducting material may be moved laterally along the surface `of the base sheet -to distort the pattern of the ultimate inlaid printed circuit. Accordingly, employment of the process described above positively assures an accurate, flush, inlaid printed circuit.

Where the ultimate inlaid circuit requires a particularly tight bond between the conducting portions i3 and the permanent base i4, this may be accomplished without regard for the firmness of the bond between the permanent -base lf3 and the base sheet il. For example, a relatively rigid `permanent base l may be coated with an eiective adhesive, such as an epoxy resin. This may be pressed against the assembly of Fig. 5 as illustrated in Fig. 6, such that the adhesive coating flows over and rmly contacts all areas of the conducting material i3 and the exposed portions of the lower surface of the base sheet il. An adhesive of the character suggested above will provide a very tight bond between the permanent base is and the conducting material I3. Naturally, a very tight bond will also result between the permanent base Eli and the base sheet H, whereby it would 4be impossible to strip the inlaid circuit from the kbase sheet. Such an adhesive may, for example, have an adherence for the conducting material and for the base sheet of a value on the order of '5000 pounds per square inch, an adherence many times too large to permit stripping of the inlaid circuit from the base sheet. This rm bond, as in the case of the firm bond between the conducting material !3 and the base sheet El, has `no effect on the process since the'base sheet H is removed by a solvent.

In accordance with the invention, then, the base sheet H is readily removed without regard to the firmness of the bond between the base sheet and any portion of the inlaid circuit. The yinvention therefore permits the firmest possible bond between the conducting material and the base sheet Vil and the rmest possible bond 'between the permanent vbase -I l 'and the assembly of Fig. 5, while permitting ready removal of the base sheet in 'spite-'of vvvthese firrn'fbonds. 1

Many alternatives of `the .basic process may be employed while still retaining the basic advantages of the preferred vembodiment described above. For example, the photosensitive vresist of the preferred embodiment .may vbe replaced by a resist which is applied to the base sheet 11 by printing, rolling, or screening. The photosensitive coating is preferred because greater accuracy is possible with this .form of resist Athan with printing, rolling, or screening. The latter methods are generally somewhat less expensive, however, and may therefore be preferred 'where great accuracy in the circuit .is `not required.

Various materials may be employed for the 'resist, especially where -it kis applied by printing, rolling, yor screening. By way of example, `.the resist may be of the same material as, or may be of a material compatible with, the material femployed as the permanent base. In this case the resist need not be removed from the base sheet ll after the deposition of the conducting material. Instead, the material for the permanent base vmay be applied against the exposed surface of 'the conducting material and against the surrounding area under conditions of suitable temperature and pressure. The resist material then ultimately Iconstitutes a portion of the permanent base, and more speciiically constitutes that *portion which appears l`on the flush surface ofthe completed inlaid circuit. Any form vof resist may, of course, be usedin'this manner, but where the photosensitive resist of the preferred embodiment is employed, this material is a relatively unstable material to use as a part of the permanent base.

Where the conducting portion of the inlaid circuit will not be subjected to severe abrasion or temperature, a soft (low melting temperature) metal may be employed therefor. In this case lthe conducting metal rather than being electrolyt'ically deposited, may be applied by dipping the assembly of Fig. 3 into a bath of the desired molten metal, such as a tin lead solder, or the molten metal may be applied by spraying. This method of applying the conducting kmaterialto the base sheet il maybe somewhat less expensive than the 'electrolytic process and would be desirable for that y*reason where the metals which might be so applied are satisfactory in View of the intended use-of the ultimate inlaid circuit.

Zinc may be employed for the base sheet ,Il in which case it may be removed in the final step Aby a 10% solution of nitric acid. `Aluminum may valso be employed for the base sheet, in which ycase the aluminum sheet may rbe removed in the lnal stepiby a strong lye solution (on the order oft6 -ounces of lye per gallon of water). If aluminum is to be used for the base sheet l l, kand if 1a .photosensitive resist is to be employed such as 'that described in the preferred embodiment of the in,- vention, it is preferable that the residue coating '1'2 be removed, as in Fig. 5, by a'nagent other than the lye solution suggested, since the llye would attack the aluminum base sheet. Theresidue coating may, for example, be removed .in this case by trisodium phosphate.

Still further, if the intended use of the inlaid circuit makes it necessary-to employ nickel as .the conducting material, a copper base sheet il may be employed. In this case the base sheet .may be removed in the final step of the process by ychromic acid, which latter .is .substantially inert toward nickel.

In accordance with several embodiments of the invention, described below, no .resist need be ap.-

plied to the base sheet II prior to the bonding thereto of the conducting material of the inlaid circuit. For example, the base sheet I I may have a suitable soldering iiux applied thereto in the pattern of the desired circuit. The base sheet I I may then be dipped in a bath of molten solder, with the result that the solder will adhere to the base sheet in the desired pattern, that is, where the ux was previously applied. The flux may be applied by any suitable method such as painting, rolling, screening, or printing. Another alternative which would require no resist, as such, involves the spraying of molten metal against the base sheet II through a stencil. Still another possibility involves the selective application of molten solder to the base sheet by printing or rolling. In all cases, of course, the base sheet II would ultimately be removed by a solvent which is substantially inert toward the materials of the inlaid circuit. While none of these three methods would produce a printed circuit of extreme accuracy, they illustrate that the invention is not limited to the use of a resist.

In the various embodiments described above the solvents recommended for removing the base sheet II have substantially no eiect on the materials of the inlaid circuit. However, in the interest of economy or rapidity of action it may be desired to use some certain solvent in spite of its having appreciable effect on some part of the inlaid circuit. For example, a Weak solution of ferrie chloride might be used to remove an aluminum base sheet I I where copper is used for the conducting portion of the inlaid circuit. The solvent would have appreciable eiect on the copper but would act so much faster on the aluminum than on the copper that careful use of such a solvent with these materials would be practicable if the intended use of the inlaid circuit would permit some pitting of the copper. Accordingly, where it is stated herein that the solvent for the base sheet I I is substantially inert toward the materials of the inlaid circuit, it is to be understood that the degree of inertness need be only such that the iinal product is acceptable for the use intended.

The above suggested alternatives to the preferred embodiment of the invention are far from being all inclusive and are intended only to illustrate that the invention lends itself readily to many embodiments Without departing from the spirit of the invention. More specically, each of the alternatives suggested above permits the removal of the base sheet II by a solvent which is relatively inert toward the various materials of the ultimate inlaid circuit. This step of the process permits the tightest possible bond between the materials of the inlaid circuit and the base sheet I I, whereby it is assured that the conducting elements of the inlaid circuit will cling securely to the base sheet I I during the application of the materials which are to form the permanent base.

While particular embodiments of the invention have been shown, it Will be understood, of course, that the invention is not limited thereto since many modications may be made, and it is, therefore, contemplated to cover by the appended claims any such modifications as fall within the true spirit and scope of the invention.

The invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

l. A process for producing an inlaid circuit which comprises, applying metal in a desired circuit pattern to one surface of a metal sheet in metal-to-metal contact therewith, applying an insulating material in plastic form against said applied metal and against the adjacent areas to form an inlaid circuit, and removing said metal sheet by treating With a solvent for such sheet which is substantially inert toward the materials of Said inlaid circuit.

2. A process for producing an inlaid circuit which comprises, applying conducting material in a desired circuit pattern to one surface of a metal sheet, applying an insulating material in plastic form against said conducting material and against the exposed portion of said one surface of said sheet, and removing said sheet to which said conducting material and said insulating material are applied by treating with a solvent for said sheet which is substantially inert toward said conducting material and said insulating material.

3. A process for producing an inlaid circuit which comprises, applying a nonconducting material to one surface of a metal sheet in the negative of a desired circuit pattern, applying conducting material by electrolysis to the exposed portions of said one surface of said metal sheet, applying an insulating material in plastic forni against said conducting material and against the adjacent areas to form an inlaid circuit, and removing said metal sheet by treating With a sole vent for said sheet Which is substantially inert toward the materials of said inlaid circuit.

4. A process for producing an inlaid circuit which comprises, applying a nonconducting material to one surface of a metal sheet in the negative of a desired circuit pattern, applying conducting material by electrolysis to the exposed portions of said one surface of said metal sheet, removing said nonconducting material by treating with a solvent for said material which is substantially inert toward said sheet and said conducting material, applying an insulating material in plastic form against said conducting material and against the exposed portion of said one surface of said sheet, and removing said sheet by treating with a solvent for said sheet Which is substantially inert toward said conducting material and said insulating material.

5. A process for producing an inlaid circuit which comprises, applying a nonconducting material to one surface of @a metal sheet in the negative of a desired circuit pattern, applying conducting material by electrolysis to the exposed pattern of said one surface of said metal sheet, applying an insulating material in plastic form against said conducting material and against said nonconducting material, and removing said metal sheet by treating with a solvent for said sheet which is substantially inert toward said conducting material, said nonconducting material and said insulating material,

6. A process for producing an inlaid circuit which comprises applying a nonconducting material to one surface of a metal sheet in the negative of a desired circuit pattern, bonding metal to the exposed portions of said one surface of said metal sheet in metal-to-metal contact therewith, applying an insulating material in plastic form against said bonded metal and against the adjacent areas to form an inlaid circuit, and removing said metal sheet by treating with a solvent for said sheet which is substantially inert toward the materials of said inlaid circuit.

7. A process for producing an inlaid circuit which comprises, applying conducting material in a desired circuit pattern to one chemically clean surface of a metal sheet, applying an insulating material in plastic form against said conducting material and against the exposed portion of said one surface of said sheet, and remcving said sheet to which said conducting material and said insulating material are applied by treating with a solvent for said sheet which is substantially inert toward said conducting material and said insulating material.

8. A process for producing an inlaid circuit which comprises, applying conducting material in a desired circuit pattern to one surface of a metal sheet, applying lan insulating material in plastic form against said conducting material and against the adjacent areas to form an inlaid circuit, and removing said sheet by treating with a solvent for said sheet which is substantially inert toward the materials of said inlaid circuit.

9. A process for producing an inlaid circuit which comprises, applying a nonconducting material to one surface of a metal sheet in the negative of a desired circuit pattern, applying metal to the exposed portions of said one surface of said metal sheet by treating said one surface with molten metal, removing said nonconducting material by treating with a solvent for said material which is substantially inert toward said sheet and said applied metal, applying an insulating material in plastic form against said applied metal and :against the exposed portion of said one surface of said sheet, and removing said sheet by treating with a solvent for said sheet which is substantially inert toward said applied metal and said insulating material.

10. The process of claim 3, wherein said metal sheet is copper, said conducting material is silver, and said solvent is a Water solution of ferrie chloride.

11. The process of claim 3, wherein said metal sheet is copper, said conducting material is nickel, and said solvent is chrcmic acid.

12. The process of claim 3, wherein said metal sheet is aluminum, said solvent is a lye solution, and in which said conducting material is relatively inert to said lye solution.

13. The process of claim 3, wherein said metal sheet is zinc, said solvent is a 10% solution of nitric acid and said conducting material is relatively inert to said 10% solution of nitric acid.

References Cited in the file of this patent UNITED STATES PATENTS- Number Name Date 1,804,021 Miller May 5, 1931 2,215,507 I-Iunlock Sept. 24, 1940 2,408,220 Lum Sept. 24, 194i: 2,447,541 Sabee et al Aug. 24, 1948 

1. A PROCESS FOR PRODUCING AN INLAID CIRCUIT WHICH COMPRISES, APPLYING METAL IN A DESIRED CIRCUIT PATTERN TO ONE SURFACE OF A METAL SHEET IN METAL-TO-METAL IN PLASTIC FORM AGAINST SAID APSULATING MATERIAL IN PLASTIC FORM AGAINST SAID APPLIED METAL AND AGAINST THE ADJACENT AREAS TO FORM AN INLAID CIRCUIT, AND REMOVING SAID METAL SHEET BY TREATING WITH A SOLVENT FOR SUCH SHEET WHICH IS SUBSTATNTIALLY INERT TOWARD THE MATERIALS OF SAID INLAID CIRCUIT. 